Semiconductor light sources such as light emitting diodes (LEDs) as a liquid crystal backlight or a lighting device have become prevalent. FIG. 1 is a circuit diagram of a driving circuit of an LED. The driving circuit (LED driver 90R) includes a constant current converter 100R and a control circuit 300R. The constant current converter 100R receives an input voltage VIN from a power source (not shown) by an input line 104 and boosts the received voltage VIN to supply an output voltage VOUT to an LED light source 502 as a load connected to an output line 106 and also stabilize a current (a load current or a driving current) ILED flowing in the LED light source 502 to a target value IREF. For example, the LED light source 502 is an LED string.
The constant current converter 100R is, for example, a boost converter, and includes a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1, an inductor L1, and a detection resistor RCS.
As a method for changing a quantity of light (brightness) of the LED light source 502, analog dimming and pulse width modulation (PWM) dimming have been known. FIG. 2 is a waveform view illustrating analog dimming and PWM dimming.
The analog dimming changes the amplitude (current amount) of the driving current ILED. For the analog dimming, an error amplifier 304 and a duty controller 306 are provided. The current ILED flowing in the LED light source 502 flows into the detection resistor RCS to generate a voltage drop in proportion to the current ILED of the detection resistor RCS. The voltage drop as a detection voltage VCS is input to a current detection (CS) terminal of the control circuit 300R. An analog dimming voltage VADIM representing the target value IREF of the load current ILED from an external host processor 400 is input to an analog dimming (ADIM) terminal of the control circuit 300R. The control circuit 300R generates a driving pulse SDRV whose duty ratio is adjusted such that the detection voltage VCS is identical to the analog dimming voltage VADIM, and drives a switching transistor M1.
The error amplifier 304 amplifies an error between the detection voltage VCS and the analog dimming voltage VADIM to generate a feedback signal VFB corresponding to the error. For example, the error amplifier 304 includes a transconductance amplifier (gm amplifier), and a resistor RFB and a capacitor CFB for phase compensation connected to an output thereof. The duty controller 306 is a so-called pulse modulator, and generates the driving pulse SDRV having a duty ratio based on the feedback signal VFB. The driver 308 switches the switching transistor M1 according to the driving signal SDRV.
In this constant current converter 100R, the feedback signal VFB is applied such that the following relational expression is established.ILED×RCS=VADIM 
Thus, the load current ILED is stabilized to the target current amount IREF which is in proportion to the analog dimming voltage VADIM.
Next, the PWM dimming will be described. In the PWM dimming, an effective light quantity is changed by changing the illumination time of the LED light source 502. A dimming pulse SPWMIN from the host processor 400 is input to a PWMIN terminal. The dimming pulse SPWMIN has a duty ratio corresponding to a target light quantity of the LED light source 502. A driver 330 switches a PWM dimming switch M2 according to the dimming pulse SPWMIN.
Jitter is superimposed on the dimming pulse SPWMIN generated by the host processor 400. As illustrated in FIG. 2, a timing of a positive edge/negative edge of the dimming pulse SPWMIN is fluctuated randomly or periodically on the time axis due to the jitter, causing an error of the duty ratio (pulse width). When a duty ratio is large so the brightness of the LED light source 502 is high, the influence of the jitter may be neglected. However, when the duty ratio is small so the brightness of the LED light source 502 is low, the fluctuation of brightness resulting from the jitter, that is, flickering, is visible to human beings. In particular, since the eyes of humans have logarithmic sensitivity, low brightness and a small fluctuation in brightness may be easily recognized.
Further, in order to solve this problem, it is necessary to increase the clock accuracy of the host processor 400 for generating the dimming pulse SPWMIN, but in this case, costs are increased. Here, the jitter is taken as an example as a factor of flickering, but flickering may also occur due to other factors, for example, noise. This problem must not be recognized by a range of common general knowledge in the art to which the present disclosure pertains, and it is recognized by the inventor of the present disclosure independently.